1. Field of the Invention
This invention relates to modular medical implants and more specifically to multi-component prosthetic implants where interconnecting components are made of different metallic alloys and ceramics. More specifically, the invention provides a dual composition sleeve or interpositional device which, when interposed between components of a medical prosthesis, reduces or eliminates electrochemical activity and resultant galvanic corrosion that may occur between the two modular components in the body.
2. Description of the Related Art
An increasing number of hip replacement prostheses are now based on modular designs. Typically, in such a design, the femoral head is a separate component that is press-fitted and locked onto the neck portion of the hip stem. The heads may be of inert alumina or zirconia ceramic but the most popular heads are those fabricated from a cobalt, stainless steel, or titanium alloy. Commonly, for non-cemented cases, the hip stem is made of the more flexible titanium alloys and the femoral head is made of a more abrasion resistant alloy such as a cobalt alloy. It is believed that this system gives the best long-term (non-cemented) performance. However, it has now been found that when such dissimilar alloys are used to form modules of the hip prosthesis then galvanic corrosion may contribute to dissolution of metal at the metal interface. The body fluids act as an electrolyte for the conduction of electrons and an associated low amperage current which results in electro-chemical corrosion of the alloys which may loosen the connection, and release metal ions into the body tissue.
In order to overcome this contribution of metal ions from galvanic corrosion in such modular press-fit connections, one might substitute a ceramic femoral head for the abrasion resistant cobalt alloy femoral heads. However, such a substitution, while solving the potential problem of galvanic corrosion, presents a potential new problem associated with relatively brittle ceramics: catastrophic failure of the ceramic head. For instance, U.S. Pat. No. 4,921,500 points out that when ceramic materials are used in a femoral head component, they are more "frangible" than metals and it has been found that the standard taper connections, such as the Brown and Sharpe taper, establishes hoop stresses within the ceramic femoral. When these hoop stresses exceed the strength of the ceramic material in tension, the solid ceramic head may fail with catastrophic results. To overcome this stress problem, the '500 patent discloses as its invention an adaptor fabricated of biocompatible alloy, such as titanium, having specified inner and outer taper for interposing between the borehole in the femoral head and the post on the neck of the femoral stem component. Such an adaptor is said to enable the securement of the ceramic femoral head to the post of conventional metallic femoral stems, providing a secure fit without the generation of excessive hoop stresses which could damage the ceramic femoral head.
The use of hard metallic femoral heads is however still preferred because of their resistance to damage caused by impact, greater ease of fabrication, and lower modulus of elasticity as compared to ceramics.
U.K GB 2 230 192 shows a coupler for a prosthesis having a neck made of a first material and having a first taper, and a head made of a second material with a blind bore of a second taper. The coupler has an outer surface with a taper corresponding to the second taper for cooperating with the borehole and an inner surface with a taper corresponding to the first taper for cooperating with the neck. The outer surface texture of the coupler has a first degree of coarseness for tight coupling with the inner surface of the borehole in the femoral head component. Further, the inner surface of the coupler has a surface texture of a second degree of coarseness, different from the first degree of coarseness, for tight coupling with the outer surface of the neck of the femoral stem. In the preferred embodiment, the interpositional coupler is fabricated of a titanium alloy, Ti-6Al-4V. The patent does not anywhere address the problem of galvanic corrosion when two components of a prosthesis, each having a different metallic composition than the other, are joined together in the patient's body. Indeed, this problem has only recently come to light. It is now known that after 40 months of implantation galvanic assisted corrosion occurs in 100 percent of the cases where a cobalt alloy femoral head is coupled to a Ti-6Al-4V hip stem. Although contribution from galvanic corrosion may be small for these metal combinations using proper design, it is yet desirable to reduce galvanic effects further. When the same material is used for both stem and head, no galvanic pitting corrosion is found. However, most surgeons prefer to use a harder, more abrasion resistant, metal alloy, such as the cobalt alloys, for the femoral head and a more ductile alloy, Ti-6Al-4V, for the hip stem.
What is yet needed is a method for the elimination of galvanic assisted corrosion that may occur in the body when a first component of a prosthesis having a first metal alloy composition is fixedly or removably attached to a second component of a prosthesis having a second, different metal alloy composition.